1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus which forms images using a two-component developer including a toner and a carrier. In addition, the present invention also relates to an image forming method.
2. Discussion of the Background
Electrophotographic image forming methods are well known and typically include the following processes.    (1) An electrostatic latent image is formed on an image bearing member (latent image forming process);    (2) The electrostatic latent image is developed with a developer including a toner to form a toner image on the image bearing member (developing process);    (3) The toner image is transferred onto a receiving material optionally via an intermediate transfer medium (transfer process); and    (4) The toner image on the receiving material is fixed thereto, for example, upon application of heat and pressure thereto (fixing process).
The developing process is typically performed using a two-component developing method or a one-component developing method. Two-component developing methods use a two-component developer including a toner and a carrier. Specifically, an electrostatic image formed on an image bearing member is rubbed with a magnetic brush of a two-component developer, which includes a toner and a magnetic carrier and which is formed on a developer bearing member, resulting in formation of a toner image on the image bearing member. The magnetic brush is formed by a magnet (i.e., a magnetic pole) provided in the developer bearing member. Such two-component developing methods are typically used for electrophotographic image forming apparatuses because of being able to easily produce color images.
When the toner concentration (i.e., the weight ratio (T/C) of the toner to the carrier) is too high in a two-component developer, a background development problem in that the background portion of an image is soiled with toner particles, and a problem in that the resolution of fine images deteriorates tend to occur. In contrast, when the toner concentration is too low, a low density problem in that produced toner images have a low image density, a carrier adhesion problem in that not only toner particles but also carrier particles are adhered to an electrostatic latent image formed on an image bearing member tend to occur. Therefore, it is important to control the concentration of the toner in a two-component developer so as to fall within a proper toner concentration range. In order to control the concentration of the toner, a toner concentration controlling method such that the toner concentration in a developer is checked, and a toner replenishing operation is properly performed depending on the toner concentration is used.
It is important for an image forming apparatus that produced images have a constant image density. The image density of images produced by an image forming apparatus mainly depends on the developing ability of the developing device of the image forming apparatus. Specifically, the developing ability of a developing device is defined as the weight of a toner adhered to an electrostatic image having a predetermined potential by the developing device. The developing ability depends on the concentration of the toner in the developer used; developing conditions such as the development potential which is the difference between the potential of an electrostatic image and the development bias applied to the developer bearing member; and the quantity of charge of the toner used. It is well known that the developing ability of a developing device is represented by the slope (development gamma characteristic γ) of a relationship equation between development potentials and weights of toner particles adhered to an electrostatic image.
Thus, the image density of images produced by an image forming apparatus depends on the developing ability of the developing device thereof. Therefore, the image density cannot be controlled by merely controlling the concentration of the toner in the developer used. Specifically, the development conditions such as development potential can be controlled relatively easily, but it is difficult to control the charge quantity of the toner used for development. Therefore, it is difficult to control the developing ability (i.e., the image density) by merely controlling the development conditions and the toner concentration.
For example, when images with a low image area proportion are produced, only a small amount of toner particles are consumed for forming the images. Therefore, a small amount of new toner particles are replenished to the developing device. In other words, almost all the toner particles in the developing device are staying therein for a long period of time while agitated. Therefore, almost all the toner particles are fully charged and have a desired charge quantity. In this case, the developing ability of the developing device is relatively low. In contrast, when images with a high image area proportion are produced, a large amount of toner particles are consumed for forming the images, and therefore a large amount of new toner particles are present in the developing device, which have a charge quantity lower than the desired charge quantity. In this case, the developing ability of the developing device is relatively high.
In recent years, a need exists for small-sized image forming apparatus and developing devices. Therefore, the amount of toner particles contained in such a small-sized developing device is small. Therefore, after production of images with a high image area proportion, toner particles in the developing device are not sufficiently charged. In this case, the developing device has a relatively high developing ability.
However, it is possible that a developing device has a higher developing ability after production of images with a low image area proportion than in a case where images with a high image area proportion are produced. For example, when a toner in which an external additive is adhered to toner particles is used and the toner is agitated for a long period of time in a developing device while receiving a high stress, problems in that the external additive is embedded into the toner particles or released therefrom tend to occur. In this case, the fluidity and charging ability of the toner deteriorate, and therefore the toner particles staying in the developing device for a long period of time have a low charge quantity. Therefore, after production of images with a low image area proportion, a large amount of toner particles having a low charge quantity are present in the developing device, and the developing device has a high developing ability. In contrast, after production of images with a high image area proportion, the developing device has a low developing ability because of including a large amount of new toner particles, which have good fluidity and charging ability. In this case, the new toner particles can be easily charged and have a desired charge quantity. Therefore, the developing device has a relatively low developing ability.
Thus, the content of new toner particles in a developing device changes depending on the image area proportion of the produced images, resulting in change of the developing ability of the developing device. Therefore, even when the development conditions of a developing device and the toner concentration are controlled so as to be constant, the developing ability of the developing device cannot be controlled so as to be constant, resulting in occurrence of a problem in that images with a constant image density cannot be produced.
In attempting to solve the problem, published unexamined Japanese patent applications Nos. 57-136667 and 02-034877 have disclosed image forming apparatuses having a toner concentration detecting device. In the image forming apparatuses, the toner concentration detecting device detects the concentration of toner in the two-component developer contained in the developing device and outputs the toner concentration data. The image forming apparatuses compare the toner concentration data with the target of the toner concentration to control the toner replenishing device on the basis of the comparison result so that the concentration of toner in the two-component developer contained in the developing device approaches the toner concentration target. In addition, the image forming apparatuses form a reference toner image on a non-image area of the image bearing member thereof and measure the image density of the reference toner image to correct the toner concentration target on the basis of the image density of the toner image.
By using this method, it is possible to produce images having a constant image density for a while after correction of the toner concentration target. Therefore, by performing the reference toner image formation operation and the toner concentration target correction operation at regular intervals, it will be possible to produce images having a constant image density. However, this method has a drawback in that whenever the toner concentration correction operation is performed, a reference toner image has to be formed, resulting in increase of the toner consumption.
Because of these reasons, a need exists for an image forming apparatus which can produce images having a constant image density without increasing toner consumption.